The present disclosure generally relates to biocides, and more particularly, to the use of 3,5-dimethyl-1,3,5-thiadiazinane-2-thione in gas and oil field well stimulation fluids.
After a well is drilled into a subterranean geological formation that contains oil, natural gas, and water, every effort is made to maximize the production of the oil and/or gas. To increase the permeability and flow of the oil and/or gas to the surface, the drilled wells are often subjected to well stimulation. Well stimulation generally refers to several post drilling processes used to clean the well bore, enlarge channels, and increase pore space in the interval to be injected thus making it possible for fluids move more readily into the formation.
A typical well treatment process generally includes pumping specially engineered fluids at high pressure and rate into the subterranean geological formation. The high-pressure fluid (usually water with some specialty high viscosity fluid additives) exceeds the rock strength and opens a fracture in the formation, which can extend out into the geological formation for as much as several hundred feet. Certain commonly used fracturing treatments generally comprise a carrier fluid (usually water or brine) and a polymer, which is also commonly referred to as a friction reducer. Many well stimulation fluids will further comprise a proppant. Other compositions used as fracturing fluids include water with additives, viscoelastic surfactant gels, gelled oils, crosslinkers, oxygen scavengers, and the like.
The well treatment fluid can be prepared by blending the polymer with an aqueous solution (sometimes an oil-based or a multi-phase fluid is desirable); often, the polymer is a solvatable polysaccharide. The purpose of the polymer is generally to increase the viscosity of the fracturing fluid that aids in the creation of a fracture; and to thicken the aqueous solution so that solid particles of proppant can be suspended in the solution for delivery into the fracture.
The polymers used in well treatment fluids are subjected to an environment conducive to bacterial growth and oxidative degradation. The growth of the bacteria on polymers used in such fluids can materially alter the physical characteristics of the fluids. For example, bacterial action can degrade the polymer, leading to loss of viscosity and subsequent ineffectiveness of the fluids. Fluids that are especially susceptible to bacterial degradation are those that contain polysaccharide and/or synthetic polymers such as polyacrylamides, polyglycosans, carboxyalkyl ethers, and the like. In addition to bacterial degradation, these polymers are susceptible to oxidative degradation in the presence of free oxygen. The degradation can be directly caused by free oxygen or mediated by aerobic microorganisms. Thus, for example, polyacrylamides are known to degrade to smaller molecular fragments in the presence of free oxygen. Because of this, biocides and oxygen scavengers are frequently added to the well treatment fluid to control bacterial growth and oxygen degradation, respectively. Desirably, the biocide is selected to have minimal or no interaction with any of the components in the well stimulation fluid. For example, the biocide should not affect fluid viscosity to any significant extent and should not affect the performance of oxygen scavengers contained within the fluid. The oxygen scavengers are generally derived from bisulfite salts.
Other desirable properties for the biocide are (a) cost effectiveness, e.g., cost per liter, cost per square meter treated, and cost per year; (b) safety, e.g., personnel risk assessment (for instance, toxic gases or physical contact), neutralization requirements, registration, discharge to environment, and persistence; (c) compatibility with system fluids, e.g., solubility, partition coefficient, pH, presence of hydrogen sulfide, temperature, hardness, presence of metal ions or sulfates, level of total dissolved solids; (d) compatibility with other treatment chemicals, e.g., corrosion inhibitors, scale inhibitors, demulsifiers, water clarifiers, well stimulation chemicals, and polymers; and (e) handling, e.g., corrosiveness to metals and elastomers, freeze point, thermal stability, and separation of components.
Current well stimulation fluids generally employ either glutaraldehyde or tetra-kis-hydroxymethyly-phosphonium sulfate (THPS) to control bacterial contamination. Glutaraldehyde can be problematic because it is hazardous to handle and has environmental concerns. Moreover, it has been observed that glutaraldehyde can deleteriously affect the fluid viscosity of the well treatment fluid at elevated temperatures; temperatures that are commonly observed during use of the well treatment fluid. This can be problematic in fracturing applications since the higher maintained fluid viscosity down hole could hinder flow back. In addition, glutaraldehyde has been shown to negatively impact the behavior of the oxygen scavenger.
With regard to THPS, although it has been shown to perform better than glutaraldehyde with respect to interaction with the oxygen scavengers, THPS has been found to interact with the polymer and limit viscosity development when added pre-inversion and post-inversion. That is, THPS has been observed to interact with the polymer during shear and significantly reduce fluid viscosity.
Thus, there remains a need for a more versatile biocide for use in well stimulation fluids that can effectively control bacterial contamination and have minimal interaction with the polymer and/or oxygen scavenger.